sofie callibration and testing

SOFIE Calibration Sources

Solar Observations from SDL

One aspect of SOFIE calibration and testing will involve measurements of solar intensity from SDL in Logan, Utah. Figure 1 shows an example of atmospheric transmissions calculated for solar rays impinging the earth's surface, considering a variety of view angles and ground elevations. These results illustrate that the atmosphere is increasingly opaque as view angle increases from zenith, and towards lower elevations. Table 1 lists atmospheric transmissions calculated for each SOFIE band assuming an overhead sun viewed from Logan. While a few channels are completely opaque (small transmissions), many will receive a substantial signal when viewing the sun from Logan.

Figure 1. Atmospheric transmission versus view angle calculated for the SOFIE 2.8 um CO2 channel weak band (band 8). Curves are presented for various observation altitudes (Logan is 1.38 km).

Table 1. Atmospheric transmission computed for a zenith solar view from Logan (4,535 ft or 1.382 km altitude), assuming a standard atmosphere. Molecular (Rayleigh) extinction was included in the calculations for bands 1 - 4 and the sun sensor.
Band	Target	Transmission
1	O3 (s)	3e-5
2	O3 (w)	0.52
3	PMC	0.98
4	PMC	0.99
5	H2O (w)	0.70
6	H2O (s)	9e-9
7	CO2 (s)	1e-9
8	CO2 (w)	0.30
9	PMC	0.28
10	PMC	0.41
11	CH4 (s)	0.62
12	CH4 (w)	0.90
13	CO2 (s)	3.1e-4
14	CO2 (w)	0.85
15	NO (w)	0.58
16	NO (s)	0.12
sun sensor (705 nm)		0.94

Solar Simulator Tests

The brightest stimulus available for ground testing is a solar simulator that achieves 3000K. This is low when compared to the sun which behaves like a blackbody at 5780K. Figure 2 compares the irradiance spectrum from a solar model to spectra from the Planck function for 5780 and 3000K. Note that the Planck expression is a good approximation to the overall shape and magnitude of the solar spectrum, but does not contain the fine structure apparent in the model. The solar simulator irradiance spectrum should be very close to that of the 3000K Planck curve in Figure 2.

Figure 2. Irradiance spectrum from a solar model compared to spectrum from the Planck function for 5780 and 3000K.

Figure 3 (top) shows the irradiance values that result from integrating the Planck function or the solar model, over the spectral response functions for each SOFIE band. Irradiances determined from the Planck function at 5780K closely match results based on the solar model, except at the two shortest wavelengths where the solar model gives roughly 50% less irradiance. The band integrated irradiances for 3000K are less than the solar model values, ranging from a factor of 0.45 at the longest wavelength to 0.0007 at the shortest. The SOFIE pre-amp signal gains will be set such that the unattenuated sun viewed from orbit results in a signal of 3.6 volts entering each balance attenuator. These settings will initially be predicted using the SOFIE radiometric model. If the desired settings are achieved, then the signals resulting from a 3000K stimulus should be as shown in Figure 3 (bottom).

Figure 3. Top: Irradiance from a solar model and the Planck function at two temperatures, integrated over the spectral response functions for each SOFIE band. Bottom: The A/D counts resulting from a 3000K stimulus, assuming the on-orbit unattenuated solar signal into the balance attenuators is 3.6 volts.